Cooled flame tube



Nov. 1, 1960 F. J. BAYl-.EY

cooLED FLAME TUBE Filed Sept. 15, 1952 2 Sheets-Sheet Il 0 oo ww om ovmww wm o v, o U oo 00 oo oo oo oo oo oo o o no oo oo ,oo oo oo 0 00 0 O000 oo oo nu o o o o ow ow ow ow ow ow Inven ,L'fAtrarneyl 2 Sheets-Sheet2 OOOOOOOO LUI inve/:for

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assisi retreated Nov. 1, i960 COLED FLAME TUBE Frederick .lohn Bayley,Monkseaton, England, assigner to Power .lets (Research and Development)Limiti-ea, London, England, a company of Great Britaan Filed Sept. 15,1952, Ser. No. 309,718

'Claims priority, application Great Britain Sept. 24, 1951 11 Claims.(Cl. 60-39.65)

This invention relates to combustion apparatus and is particularlyconcerned with the problem of cooling the walls of flame tubes and likedevices enclosing a space in which combustion takes place. It isparticularly applicable to combustion chambers, such as those used in 2gas turbine plants, which comprise a flame tube mounted within anenclosing air casing, part of the air supply entering the ame tube atone end as primary air, and part ilowing around the outside thereof andentering further downstream `as secondary and/ or cooling air, or insome cases mixing with the hot gases leaving the flame tube.

The air flowing around the outside of the ilame tube has some coolingeffect. This may be supplemented by iutroducing a fairly small amount ofair into the upstream end of the llame tube so that it flows at the samespeed as the hot gases `along the wall `as a thin film or layer, thusprotecting the wall from the direct effect of the hot gases. A system ofthis type is described in United States patent to Lloyd et al., No.2,664,702, issued January 2, 1954. The effectiveness of this method islimited by the di'lculty of maintaining this cooling layer for anadequate distance along the length of the ame tube, and it isaccordingly necessary to renew the layer at frequent intervals.

Accordingly the present invention provides combustion apparatuscomprising a llame tube and an enclosing jacket annularly spaced fromthe peripheral wall thereof, the space between the flame tube and thejacket being divided into a plurality of axially successive sections,and each section being open at one end to the exterior of the jacket andat the other end to the interior of the llame tube to form a passage forthe ow of cooling air therethrough. K

So as to form the thin cooling layer of air next to the Wall of theflame tube, the passages for cooling air are preferably so dimensionedthat the cooling air enters with l a radial velocity substantially equalto and in any case not greater than the axial velocity of the gaseswithin the flame tube.

The invention will be more fully described with reference to theaccompanying drawings, of which:

Figure l is a longitudinal section of one embodiment of the invention.

Figure la is a fragmentary perspective View of the flame tube shown inFigure l.

Figure 2 is a half section on the line Il-II in Figure l.

Figure 3 shows a detail of the embodiment of Figure l.

Figure 4 is a longitudinal section of a second embodiment.

Referring to Figure l a combustion chamber for a gas turbine plantcomprises an air casing 1 and a llame tube 2 within it. The upstream endof the air casing is generally frusto-conical in shape, being divergentinthe downstream direction and is provided with a ilange 22 forattachment to the duct carrying the air supply, for example, from acompressor. The downstream end of the air casing is also somewhattapered, being convergent in the downstream direction and may iit into achute leading to the turbine nozzle. The flame tube Z is similarlyformed with divergent and convergent portions at its upstream anddownstream ends respectively. It is supported inthe air casing 1 bybrackets 3 towards its upstream end and by a frusto-conical flangenumber 4 towards :its downstream end.

Mounted within the upstream end of the flame tube 2 is a generallyfrusto-conical baffle 5 supported therein by brackets 5a, so that itswider end forms with thetlarne` tube wall an annular gap 13 for coolingair. A fuel injector 6 extends into the upstream end of the baie, beingsupported therein by swirl vanes 7, and is connected to a fuel supply bypipe 8. The baille is perforated at 9, 10 for the admission ofcombustion air to the space within the baie. Y

Surrounding the peripheral wall of the llame tube 2 is a jacket 11. Thejacket is carried on a number of annular bracket members 12 (shown inmore detail in Figure 3) which also constitute walls dividing theannular space between the ame tube and the jacket into a number ofaxially successive annular sections 14a, 14h, 14e 14n. The flame tube isformed with a double row of small apertures 15a, 15b, etc. at theupstream end of each of these sections, while the jacket is formed witha ring of small apertures 16a, 16h, etc. at the downstream end of eachsection.k The apertures and annular sections thus constitute a number ofseparate annular passages for cooling air. t

The jacket 11 does not extend the full length of the flame tube, andbeyond its downstream end the llame tube is foimed with a number ofcomparatively large elongated apertures 17 for the admission of dilutionair. The tapered portion at .the downstream end of the llame tube isalso formed with two axially spaced series of small holes 18a, 18h, eachseries consisting of two rings of holes similar to the holes 15a, 15b,etc. The annular bracket member 4 is perforated at 19 to allow air toenter the space between the downstream ends of the air casing 1 and thellame tube 2 to flow through the holes 18a, 18h as cooling air.

Referring to Figure 2, the annular sections 14a, 14b,

etc. between the flame tube 2 and jacket 11 are dividedcircumferentially by longitudinally extending ns 20 attached to theflame tube walls and forming secondary cooling surfaces.

Figure 3 shows on an enlarged scale one of the bracket memberssupporting the jacket 11 on the llame tube 2. The brackets areapproximately Z-shaped in section, and one arm 12a is attached, forexample by welding 12e, to the flame tube wall. The other arm 12b is notattached to [the jacket which is free to slide thereon. Thus ifdifferential thermal expansion should arise through the llame tubeexpanding to a greater extent than the jacket, the centralfrusto-conical part of the bracket will be able to bend over slightly toallow for the reduced radial dimension of the annular space. The arm 12band the central portion of the bracket are formed with a number ofnarrow axially extending slits 12C terminating in holes 12d tofacilitate this bending.

In operation, air enters the air casing 1 and part is divided off andenters the upstream end of the flame tube 2. Of this part of the air,some passes into the interior of the baille 5 through the swirl vanes 7and the perforations 9 and 10 as primary combustion air. The baflieforms a stabilised combustion zone in ywhich the fuel from the injector6 is burnt and the resultant stream of hot combustion gases passes downthe flame tube towards the outlet at a high Velocity.

A comparatively small part of the air entering the flame tube passesthrough the accelerating passage formed between the baie 5 and theupstream part of the flame tube wall and enters the main part of theflame tube through the annular gap 13. This gap is so dimensioned thatin some particular condition of operation, for example full power, theaxial velocity of the stream of air therethrough is substantially thesame as and in any case not greater than the axial velocity o f the.Sirf-Tant .0f .GQ133- bUStiOH gases alone the me tiibe As explained inSaid United States Patent NO-Q-JOZ, the. air thus tends t form anannular layer of cool air next to the flame tube wall, thus protectingit from the full heating effect of the hot gases. As previouslymentioned however, thisy layer may riot persi-SL There its a great deal.of turbulence Within the `battle 5 and the cooling layer tends to bedrawn irit this turbulent zone, and hence its cooling eiect may notpersist very far downstream.

The par@ sf. the air not passing iutqthe 11am@ tube passesY around theoutside thereof in the conventional manner and thus has certain coolingeffect. Af comparatively small amount of this `air passes through theapertures 16a in the jacket il into the space 14ga and then through theapertures a into'the interior. of the llametube. In flowing through theYspace Mrz the air has a considerable convective dcooling effect. isintendedl that the air Oil'ente'rig the '5eme tube Shall @0W @1011s theWall thereof as `a cooling layer in the same way as the air enteringthrough the gap`13. It is found by experiment that for this purpose theair should enter through the apertures 15d with la radial velocity notgreater than the axial velocityof the hot gases. The pressure lof theair in the air casing is'higher than the pressure of the hot gases inthe flame tube, so there will be no ditiiculty in maintaining thecooling flow and the apertures 15a can be so dimensioned` to give therequired velocity. If the pressure drop causedby the apertures 15a isinsufficient, an additional pressure drop can be achieved by suitablydimensioning the apertures lla.` Alternatively or in addition the nswithin theV jacket may be spirally disposed so that the effectivecross-sectional area of the ow path for the cooling air through thesection 14a is reduced and the air consequently will achieve a greatervelocity. This arrangement will enhance the convective cooling effect ofthe air before it enters theiiame'tube, y l

To establish a satisfactory cooling layer of air within the ame tube,the air should ideally enter thel flame tube through an apertureconstituting a complete circumferentially extending annulus. In practiceit would be difficult to maintain thev accuratel dimensioning of suchanaperture dueto thermal expansion of theV flame tube and `so it ispreferred to'use two or more rows ofcircular holes, staggered withrespect to one another so thatvthey, in effect form arf-complete ring'.In the embodimentshown in Figure l, two 'rowsfare used, the holes beingpitched circumferentially andV axially one diameter apart, and the tworows'being staggered. Holes of the orderof JAG diameter are found togive satisfactory'results. In the drawings however, these holes are'shown as enlarged 'out of proportion to the rest of the assembly.

From experiment it is found that thecoolingV air as it enters the flametube produces'a considerable local cooling effect on the part of thewall immediately around the apertures 15a. The air then mixes with. acertain amount of the hot gases, consequently increasing in temperature,and then flows in a thin layer alongthe inside of the flame tube wall.Thislayer decays by 'mixing with the hot gas stream and accordinglyits`temperature gradually rises. The wall temperaure is initially lowerthan that of the cooling layer, due to the initial coolingeffect of theair,

butl its temperature rises at a greaterratey than that of thel coolinglayer so that their temperatures soon Vtend to. they same value. Thistemperaturev will.y be allowed tol rise until it reaches a valuecorresponding to` theumaxirnuml permissible temperature of the wall. Atthis point the layer is renewed by a further `stream of cooling airowing through the apertures 1Gb in the jacket, the section 14h `andmthenentering thelflame tube through. apertures 15h.v Thisw cooling layer inturn decays and is replaced byla layer formed at, the laperturesl5c,Vand so on 4 down the flame tube. Each section 14h, 14C etc. and itsapertures is designed to produce the pressure drop necessary to promotethe desired flow velocity as described above. Each section will includespirally disposed tins if necessary and the flame tube apertures willconsist of double rows of holes similar to those at 15a.

From theoretical considerations it can be sho-wn that ideally thecooling layer decays, i.e. its temperature increases, in accordance withan exponential law, and hence the axial spacing of the rows of aperturescould be calculated. This can only be used las a guide however, since itis difficult to predict the performance of the ame tube from purelytheoretical considerations particularly at the upstream end of the ametube. This is partly due to the violent turbulence associated with theprimary combustion zone which tends to break up the cooling layer morerapidly than would otherwise be the case, and partly due to the factthat the above theoretically consideration does not take into accountthe effects of radiation. When-the fuel is such as to burn with averyluminous flame, the heating effects thereof are considerable sincethe cooling layer offers little or no 'resistance to radiation. The ametube wall temperature therefore increases at a greater rate than wouldappear from theoretical considerations and it is foundnecessary toreduce the axial spacing of the injection aperture-s 15a, 15b etc. andconsequently the length of the annularsectionslia, ll-4b, etc. at leastat the upstream end` of the ame tube. Further downstream however thedisturbing factors have less influence and the axial spacing of the'apertures may be progressively increased as shown-in Figure l.

It will` thus be seen that the air entering through the ame tube Wallshas a double cooling effect. First in passing through theannularsections 14a, 14h, it has a convective cooling effect which maybe'augmented by the use of the ns 20 as secondary coolingfsurfaces.Secondly, on entering the ame tube the air-forms a protective coolinglayer next to the wall on the ame tube. This layer is renewed as itreaches a maximum permissible temperature, the air entering at 15areplacing the layer formed by gapV 13, the air entering at 15b replacingthe layer formed at 15a, and so on.

Since the cooling air within the jacket flowsin the 0pposite directionto the cooling layer Withinthe iiame tube, there is a greater tendencyfor thewail to assume a uniform temperature, sincethe hottest part ofthe layer is where the air is coolest, that is, whereit enters withinthe` jacket.

As mentioned abovethe jacket does not extend the fullV length of theflametube. The greater part of the air flowing around the outside of theflame tube enters therein as dilution air through the apertures 17downstream of the jacket. .A small portion of air passes throughmetering orifice i9 in the bracket 4 and enters the discharge end of thelflame tube as cooling air through sets ofapertures 13a, 18h. Each setconsists. of a double row of holes similar to those `at 15a, 15b. Thisair is also intended to form a cooling layer, so again the radial"elocity of the air passing through the holes should be equal to theaxial velocity of the hot gases, and the metering orifices l@ aredesigned to produce the required pressure drop. Conditions in this partof the iiame tube will not be so severe as further upstream since thegas temperature has been reduced by the introduction of dilution airthrough apertures 17.

The embodiment shown in vFigure 4 `is the same principle as theembodiment of Figure l but diifers in constructional detail. The samereference numerals have been used as far as possible for correspondingparts.

The combustion chamber comprises an outer air casing 1 having adivergent inlet portion 1a and a convergent outlet portion 1b. The flametube 2 differs from the ame tube in Figure Vl in that it has nodivergent portion at its upstream end but it has an outlet section 2afitting into its downstream end. A baffle 5 tits into the upstream ofthe assiemer flame .tube 2 defining therewith an annular gap I3 forcooling air, and fuel is supplied to the interior of the baie by pipe '8and fuel injector 6. The baffle is provided with apertures for theadmission of combustion air and swirlers 7 are located in theseapertures.

'Ihe main difference in the construction is that the jacket is made upof a number of individual jacket members 11a, 11b, 11C 11n. yEach isattached at its upstream end to a cylindrical portion 21a, 2lb, 21C Zlnof reduced diameter which is welded or otherwise secured to the llametube Wall. The upstream end of each jacket section is left open toconstitute an inlet for cooling air. members and the air is led into thellame tube through double rows of holes as in the first embodiment. Asbefore the dimensioning of the cooling air passages will be such as togive rise to a layer of cooling air next to the flame tube wall.

Downstream of the last jacket member 11n, the outlet section 2a of theflame tube wall is formed with perforation 17 for the admission ofdilution air, and the outlet portion of the ame tube is cooled inexactly the manner described with respect to Figlre l.

I claim:

l. In combustion apparatus comprising an aircasing having an air inlet,and an elongated flame tube enclosed by and spaced from the walls of theaircasing and having a tubular peripheral wall, an air inlet at one end,means for supplying fuel to said end of the flame tube for combustiontherein, and an outlet at the other end, whereby the llame tube deines apath for a stream of hot combustion gases flowing axially thereof; meansfor forming a layer of cooling air along the inside of the flame tubeperipheral wall, said means comprising a tubular jacket enclosing theflame tube for at least part of its length and radially spaced from theperipheral wall thereof to dene therewith an annular space; a wallclosing one end of said space; the flame tube peripheral wall beingformed with plurality of radially facing openings together extendingaround substantially the whole circumference of the llame tubeconnecting the space to the interior of the llame tube and the spacebeing open to the exterior of the jacket at a position axially spacedalong its length from the radially facing opening; the openings andannular space constituting a passage for the tlow of cooling air fromthe exterior of the jacket to the interior of the ame tube, and being sodimensioned in relation to the conditions inside the flame tube andoutside the jacket that the radial velocity of the stream of cooling airthrough said radially facing openings is at most equal to the Velocityof said stream of combustion gases axially of the flame tube.

2. In combustion apparatus comprising an aircasing having an air inlet,and an elongated flame tube enclosed by and spaced from the walls of theaircasing and having a tubular pheripheral wall, an air inlet at oneend, means for supplying fuel to said end of the flame tube forcombustion therein, and an outlet at the other end, whereby the llametube denes a path for a stream of hot combustion gases flowing axiallythereof; means for forming a layer of cooling air along the inside ofthe flame tube peripheral wall, said means comprising a tubular jacketenclosing the flame tube for at least part of its length and radiallyspaced from the peripheral wall thereof to define therewith an annularspace; a wall closing one end of said space; the flame tube peripheralwall being formed with plurality of radially facing openings togetherextending around substantially the whole circumference of the flame tubeat the upstream end of the annular space, in relation to the directionof flow of the combustion gases, connecting the space to the interior ofthe flame tube and the space being open at ythe other end to theexterior of the jacket; the openings and annular space constituting apassage for the ow of cooling air from the exterior of Cooling ns 20 areprovided within the jacketV 6@ the jacket to the interior of llame tube,and being so dimensioned in relation to the conditions inside the flametube and outside the jacket that the radial velocity of the stream ofcooling air through said radially facing openings is at most equal tothe velocity of said stream of combustion gases axially of the flametube.

3. Combustion apparatus according to claim 2 wherein the plurality ofradially facing openings comprise two axially spaced -rows of holesextending circumferentially of the ame Itube, the holes of the two rowsbeing staggered with respect to one another and each hole having acircumferential width-at least equal to the distance between the edgesof adjacent holes in the other row.

4. Combustion apparatus according to claim 2 further comprising iins inheat conducting relationship with the llame tube peripheral wall andextending outwardly therefrom into said annular space.

5. Combustion apparatus according to claim 4 wherein said fins arespirally arranged with respect to the ame tube axis.

6. In combustion apparatus comprising an aircasing having an air inlet,and an elongated flame tube enclosed by and spaced from the walls of theaircasing and having a tubular peripheral wall, an air inlet at one end,means for supplying fuel to said end of the ame tube for combustiontherein and an outlet at the other end, whereby the flame tube defines apath for a stream of hot combustion gases flowing axially thereof; meansfor forming a layer of cooling air along the inside of the flame tubeperipheral wall, said means comprising annular walls outside the flametube peripheral wall and defining therewith a plurality of axiallysuccessive annular spaces surrounding and spaced along the length of theflame tube; the ame tube peripheral wall being formed with plurality ofradially facing openings -together extending around substantially thewhole circumference of the flame tube connecting each annular space tothe interior of the flame tube, and each annular space being open to theinterior of the aircasing at a position axially spaced along its lengthfrom its radially facing openings; the openings and annular spacesconstituting passages for the iiow of cooling air from the interior ofthe aircasing to the interior of the flame tube and being so dimensionedin relation to the conditions inside the flame tube and inside theaircasing that the radial velocity of the streams of cooling air throughsaid radially facing openings is at most equal to the velocity of saidstream of combustion gases axially of the ame tube.

7. In combustion apparatus comprising an aircasing having an air inlet,and an elongated flame tube enclosed by and spaced from the walls of theaircasing and having a tubular peripheral wall, an air inlet at one end,means V,for supplying fuel to said end of the flame tube for combustiontherein and -an outlet at the other end, whereby the ame tube defines apath for a stream of hot combustion gases flowing axially thereof; meansfor forming a layer of cooling air along the inside of the flame tubeperipheral wall, said means comprising annular walls outside the flametube peripheral wall and defining therewith a plurality of axiallysuccessive annular spaces surrounding and spaced along the length of theflame tube; the flame tube peripheral wall being formed with pluralityof radially facing openings together extending around substantially thewhole circumference of the flame tube at the upstream end of eachannular space, in relation to the direction of flow of the combustiongases, connecting the spaces to the interior of the llame tube and eachspace being open at the other end to Ithe interior of the aircasing; theopenings iand annular spaces constituting passages for the flow ofcooling air from the interior of the aircasing to the interior of thearne tube and being so dimensioned in relation to the conditions insidethe llame tube and inside the aircasing that the radial velocity of thestreams of cooling air through said radially facing 7 openings is atmost equal to the velocity of said stream of combustion gases axially ofthe llame tube.. Y

8. In combustion apparatus comprising anv aircasing havingy an airinlet, and an elongated flame tube enclosed by and spaced from thewalls. of the aircasing and having a tubular peripheral wall, an airinlet at one end, means for supplying fuel to said end of the flametube. for combustion therein and 4arr outlet at the other end, wherebythe flame tube defines a path for a stream of' combustion gases llowingVaxially thereof; means for forming a layer of coolingV air along theinside. of the flame tube peripheral wall, said means comprising latubular jacket enclosing the flame tube for at least part of its lengthand radially spaced from the peripheral wall thereof to define therewithan annular space; walls dividing said space into. a plurality of axiallysuccessive annular sections; the flame tube peripheral Wall being formedwith plurality of radially facing openings together extending aroundsubstantially the whole circumference of the flame tube connecting eachannular section to the interior of the flame tube, and the jacket beingformed with openings' connecting each annular section to thel exteriorof the jacket at a position axially spaced along its length from itsradially facing openings; the openings and annular sections constitutingpassages for the flow of cooling air from the exterior of the jacket tothe interior of the flame tube and being so dimensioned in relation tothe conditions inside the flame tube and outside the jacket that theradial velocity of the streams of cooling air through said radiallyfacing openings is at most equal to the velocity of said stream ofcombustion gases axially of the flame tube.

9. In combustion apparatus comprising an air casing having an air inlet,and an elongated flame tube enclosed by and spaced from the walls of theaircasing and having a tubular peripheral wall, an air inlet at one end,means for supplying fuel to said end of the flame tube for combustiontherein'and an outlet at the other end, whereby the flame tube defines apath for a stream of combustion gases flowing `axially thereof; meansfor forming a layer of cooling air along .the inside of the flame tubeperipheral wall, said means comprising a tubular jacket enclosing theflame tube for at least part of its length and radially spaced from theperipheral wall thereof to define therewith an annular space; wallsdividing said space into a plurality of axially successive annularsections; the flame tube peripheral wall being formed with plurality ofradi- `ally facing openings together extending around substantially thewhole circumference of the flame tube at the upstream endv of eachannular section, in relation to the direction of'flow of the combustiongases, connecting the sections with theV interior of the llame tube, andthe jacket being formed with openings at the other end of each sectionconnecting the sections to the exterior of the jackets; the openings andannular sections constituting passages for the flow of cooling `air fromthe exterior of the jacket to the interior of the flame tube and beingso dimensioned inrelation to the conditions inside the flame tube andoutside the jacket that the radial velocity of the streams of coolingairthrough said radially facing openings is at most equal to the velocityof said stream of cornbustion gases axially of the flame tube.

10. ln combustion apparatus comprising an air casing having an airinlet, and an elongated flame tube enclosed by and spaced from the wallsof the aircasing and having a tubular peripheral wall, an air inlet atone end, means for supplying fuel to said end of the flame tube for comalayer of cooling air along the inside of the llame tube peripheral wall,said means comprising a plurality of axially successive tubular jacketstogether enclosing the flame tube for at least part of.' its length andradially spaced from the peripheral wall thereof' to define therewith aplurality of axially successive annular spaces; walls closing one end'of each said space; the other end of each jacket being Vaxially spacedfrom the adjacent end of the adjacent jacket to define an openingconnecting the space within the jacket with the exterior of the jackets;the flame tube peripheral; wal'l being formed with plurality of radiallyfacing openings together extending around substantially the wholecircumference of the-flame tube connecting each annularespace tothe'interi'or of the llame tube at a position axially spaced from saidfirst mentioned opening; the openings and annular spaces constitutingpassages for; the flow' of cooling air from the exterior of the jacketsto the' interior of the flame tube and being so dimensioned in' relationto the conditions inside the llame tube and outside the jackets that theradial velocity of the streams of cooling air through said radiallyfacing openings is at rnost equal to'the'vel'ocity of said streams ofcombustion gasesV axially of the flame tube.

111'. InV combustion apparatus comprising an aircasing having an airinlet, and an elongated flame tubeV enclosed by and spaced from thewalls of the aircasing and having a tubular peripheral' wall, an airinlet at one end, means for supplying fuel'to' said end' of the llametube for cornbustion therein, and an outletat the other end, whereby thellame tube defines apat'h for a stream of'hot combus- Vtion` gasesflowing axially thereof; means for forming a layer of coolingV air`along the inside of the flame tube peripheral wall, said meanscomprising a' plurality of axially successive tubular jackets togetherenclosing 'the flame tube for at least part of its length and radiallyspaced from the peripheralv wallthereof to' define therewith a pluralityof axially successive annular spaces; walls closing'the'upstream ends,in relation to the direction ofv flow of the combustion gases', of eachsaid' space; the downstream end of each jacket being axially spaced`from the'upstream endk of' adjacent' jacketV to define an openingconnecting they space within the jacket with the exterior of thejacketsythefflame tube peripheral wall being formed with plurality ofradially facingopenin'gs together extending around substantially' thewhole` circumference of the flame tube at the upstream end of eachannular space connectingl each annularl space Vwith the interior of theflame tube; thewopenings andl annularA spaces` constituting' passagesfor the flow of cooling air from the exterior" of the jackets to theinterior of the ilame tube andA being'so dimensioned in relation to. theconditions insidel the flame tube and outside the jackets that theradial velocity of the streams. of cooling air through' saidv radiallyfacing openingsk is.at most equal to the velocity of said stream' ofcombustion gases axially of the flame tube.

References Cited in the file of this patent UNITED STATES PATENTS2,510,645 McMahan lune 6, 1950 2,547,619 Buckland Apr. 3, 1951 2,603,064Williams Iuly 15, 1952 2,614,384 Feilden Oct. 21, 1952 2,616,258 MockNov. 4, 1952 2,664,702 Lloyd etal Jan. 5, 1954

